1. Field of the Invention
The present invention relates to an electronic device having a Ball Grid Array (BGA) solder mounting system and, more particularly, to a BGA system having an array of hollow solder balls and a method for manufacturing the same.
2. Description of the Related Art
A common mounting and electrical-connection mechanism used in integrated circuits (ICs) is a ball grid array (BGA), wherein small solder balls are placed and retained at each one of a multitude of connection pads of the IC for solder connection to a mounting substrate or planar surface having opposing metal connecting pads. When heat is applied, the solder balls liquefy and flow via a fluxing agent included in the solder compound over any exposed metal surfaces of the connection pads, thereby forming a reliable electrical connection with each mounting pad. After cooling, the hardened solder additionally provides a rigid mounting structure for mechanically retaining the IC to the substrate or planar surface.
Typically, the size of the mounting package for the IC can be reduced to the size of a semiconductor chip, thus creating a Chip Size Package or Chip Scale Package (CSP). In the CSP, unlike conventional periphery-leaded (i.e. wire-bonded) packages, an array of external terminals and the BGA solder balls are distributed over the surface of the IC to directly interconnect the package to a printed circuit board (PCB). After processing, the resulting solder structures are generally inelastic, and provide a solid mounting mechanism for the assembly.
Disadvantageously, since the material composition of the chip and the opposing epoxy-glass material of a conventional PCB can have widely mismatched Coefficient of Thermal Expansion (CTE), any thermal cycling effects, such as those normally associated with turn-on and turn-off of related circuitry, can produce differing expansion movement of the opposing planar surfaces. This movement produces lateral shearing stresses on the solder joint, which is absorbed by the solder balls, more specifically by the junction of the solder ball and the metal connecting pads. With repeated thermal cycling, metal fatigue at this junction can cause the solder structure to crack and fail, rendering an entire circuit board inoperable. In other words, when the chip heats up during use, both the chip and the board expand, and when the heat is removed, both the chip and the substrate shrink. The problem that arises is that the chip and the substrate expand and contract at different rates according the CTE, thereby stressing the interconnections or solder balls connecting them.
FIG. 1 illustrates a cross-sectional view of a conventional ball grid array (BGA) interface structure 10 having a multitude of solid solder balls 12. Structure 10 consists of upper planar element 14 having an array of first metal conductor pads 16 rigidly connected via a plurality of solder balls 12 to an array of lower planar elements 18 having a plurality of second metal conductor pads 20 so as to provide both electrical and mechanical connection between electronic circuits on each of the planar elements. Conductors are separated from lateral neighboring conductors of the array by isolation spaces 22 appropriately located on each planar element. Each of planar elements 14 and 18 can have a different CTE. However, an excessive disparity between the CTEs can produce thermal cycling failure in the form of cracking of the rigid solder joints as previously discussed.
Occasionally, conventional out-gassing of vaporized flux is not completed due to process and/or solder compound irregularities, such as insufficient wetting of the solder compound, and results in small voids being located at the conductor-solder interfaces. Such voids create an area of fatigue weakness in the resulting joining structures.
FIG. 2 illustrates a cross-sectional area of a conventional BGA structure, wherein small voids 24 due to insufficient conventional wetting are shown at the junction of one or more of the plurality of conductor pads 16 and solder balls 12 in the array. Such voids 24 are formed when each of the molten solder balls 12 wets on the associated conductor pad 16, and the flux in the solder paste compound flows outward from the center of the conductor pad 16. As the temperature of the solder compound rises further, the flux is vaporized and a major portion of the flux vapor is dissipated into the atmosphere. However, minor portion of this vapor remains trapped in solder ball 12 as it is cooled and forms the small voids 24 inside the solder ball 12.
The configurations shown in FIGS. 1 and 2 are susceptible to cracking at each joint connecting the rigid conductor pad 16 and the rigid solder ball 12 under application of opposing lateral forces on the planar elements 14 and 18. Such a failure of both the electrical connection and the mechanical mounting mechanism has heretofore precluded the use of epoxy-glass as a reliable substrate material for BGA chip attachment applications, in favor of a more expensive ceramic material which has a CTE closer to that of the chip. Similarly, attachment of ceramic carrier modules, such as hybrids and CSPs, to an epoxy-glass printed circuit board is precluded for the same reason. A detailed joint interface of solder balls used in a conventional BGA type semiconductor device mounting construction is disclosed, for example, in U.S. Pat. No. 6,122,177 and PAJ No. 1998-209591. Detailed manufacturing assembly technical reports from International Business Machines Corporation, entitled “Doubled-Sided 4 Mb SRAM Coupled Cap PBGA Card Assembly Guide,” and from Texas Instrument Corporation, entitled “MicroStar BGA Packaging Reference Guide,” would also be beneficial to the reader.
The prior art addresses the presence of these small voids in the solder balls and a resulting joint embrittlement as significant problems. A small void in this context is defined as a gaseous volumetric displacement within the interior of a solder ball due to thermal expansion (i.e. boiling) of low-temperature solder flux solvents, since such gas material will remain trapped within a cooled solder structure. Conventional solder processing typically incorporates a warm-up period to allow time for de-gassing of such solvents, thereby minimizing such voids to yield a recommended finished total gaseous volume of less than 0.1% of the total solder structure volume. Disadvantageously, such heating can prematurely dry the solder paste included in the solder ball, leading to degraded electrical connections.
Thus, conventional BGA structures are not sufficient to prevent solder cracking or the breakage of solder ball interconnection, especially when used with a chip and an epoxy-glass substrate. Therefore, what is needed is a newly designed CSP with improved interconnection reliability, especially between the chip and the PCB, and a method of manufacturing of the same.